There are hydraulically driven vehicles such as wheel hydraulic excavators in the known art that are made to travel by controlling with a control valve the flow rate and the direction of pressure oil output from a hydraulic pump that is driven by a driving motor and by driving a travel motion variable displacement hydraulic motor with the controlled pressure oil. The motor speed in this type of vehicle is controlled through the control valve which is switched as the operator steps on the accelerator pedal and also through an adjustment of the displacement volume of the travel motion motor, which is increased as the load pressure of the travel motion motor rises. Japanese Laid Open Patent Publication No. H8-270788, for instance, discloses a travel motion control apparatus for such a hydraulically driven vehicle.
The apparatus disclosed in the publication mentioned above adopts the structure described below. The operating state of a travel motion control valve is detected and also, the switch-over position of a travel motion transmission that can be switched to a high-speed setting or a low-speed setting is detected. Then, if the control valve is detected to be at a neutral position and the transmission is detected to be at the high-speed setting, the displacement volume of the travel motion motor is increased to the maximum displacement volume. As a result, the motor displacement volume increases to the maximum level to achieve a great braking force when the vehicle travels downhill with the control valve at the neutral position without the operator stepping on the accelerator pedal.
However, the apparatus disclosed in the publication does not increase the displacement volume of the travel motion motor and thus, a sufficient level of braking force cannot be achieved if the vehicle is made to travel downhill while the operator is stepping on the pedal. For this reason, the risk of an over rotation of the travel motion motor arises.